1. Industrial Field Of The Invention
The present invention relates to a light-emitting semiconductor device substrate, especially, to a light-emitting semiconductor device substrate to be used for manufacture of a light-emitting semiconductor device made of GaAlAs which has a double hetero junction structure and to a method of manufacturing the same.
2. Statement Of The Related Art
A light-emitting device of solid state includes a fluorescent substance and a light-emitting diode. The light-emitting diode mainly employs, as a light-emitting material, a III-V compound semiconductor in single-crystal form or in mixed-crystal form. The emission of light from the light-emitting diode is effected by forward biasing the pn junction, thereby injecting minority carriers through the pn junction to recombine the minority carriers with the majority carriers.
The light-emitting mechanism of a light-emitting diode allows the emitted light to be particularly high in luminance compared with that of the fluorescent materials. The light-emitting diode is suitable for space limiting emission of light or for complicated displays. In addition, the excitation energy is obtained with impression of a simple low-voltage direct current source. Further, the other features such as multicolor characteristics, high reliability, low power consumption and high speed response make it compatible with semiconductor integrated circuits. Thus, the practical use of the light-emitting diodes has been increasingly expanded.
In the initial stage, the areas of practical use of the light-emitting diodes were principally two in number; one is that of lamps to be used as point light sources for display and the other is that of displays. With technological improvement on output capacity of this kind of diode, however, the extent of applications thereof has further spread to include office automation such as light sources for facsimiles, duplicating machines, printers, outdoor displays such as traffic signals, and a communication area such as optical communications in combination with optical fibers, etc.
Thus, light-emitting diodes are now expected to enjoy a remarkably increasing demand as a solid-state light source.
By the way, initially in history, a red light emitting diode of GaP and GaAsP was employed for red light emitting purpose. A single hetero structure used of GaAlAs was thereafter developed for improving the luminance thereof. Recently the use of GaAlAs substrate and a double hetero junction structure prepared so as to effect confinement of carriers into the active layer region has enabled the luminance to be increased (1,000 mcd and more (I.sub.F =20 mA)) and the rise time to be shortened (20 nsec).
GaAlAs compound semiconductors are a mixed crystal of GaAs and AlAs, both of which are III-V compound semiconductors. Change in the AlAs mole fraction of the mixed crystal within the range of direct transition type enables light beams having wavelengths ranging from approximately 640 to 880 nm to be sent out. The longer the wavelength at which light is emitted, the higher the efficiency with which light is emitted. On the contrary, the shorter wavelengths of the emitted light are, the lower the efficiency to an utmost extent under the influence of indirect transition gets. Where the AlAs mole fraction is 0.37 or less, the band structure tends to show the characteristics of direct transition type, while the AlAs mole fraction is more than 0.37, the band structure has a tendency of indirect transition type.
A GaAlAs ultrahigh luminance red light-emitting diode with a double hetero structure is prepared as follows. A liquid phase epitaxial growth method is applied to grow, with slow cooling process, a p-type high AlAs mole fraction GaAlAs thick-film single crystal, which finally becomes a substrate, on GaAs single-crystal substrate which is manufactured by a melt growth method such as, for example, a liquid encapsulated Czochralski method. On this p-type GaAlAs thick-film single crystal layer there are sequentially grown a p-type high AlAs mole fraction GaAlAs clad layer, a GaAlAs active layer having an AlAs mole fraction which determines the wavelength of the emitted light, and an n-type high AlAs mole fraction clad layer in the order mentioned before so that a four-layer structure for a light emitting device is formed. It is to be noted that the GaAs single-crystal layer, which has been used as a substrate for initial epitaxial growth, may be removed after said p-type high AlAs mole fraction GaAlAs thick-film single crystal layer has been grown, or alternatively the substrate may be removed just prior to entering the manufacturing process of the device after the formation of the above-mentioned n-type clad layer.
The p-type high AlAs mole fraction GaAlAs thick-film single crystal layer which is the first epitaxial layer is required to be low in absorbance of light generated, to be low in specific resistance, to be good in forming ohmic contacts, and to be thick enough to make it easy to handle as a substrate in the succeeding process. For instance, the p-type GaAlAs thick-film single crystal layer which has an active impurity concentration of, for example, 3 to 5.times.10.sup.17 cm.sup.-3 in case of using Zn as a dopant, a specific resistance of 0.3 .OMEGA.cm, and a thickness of 150 to 200 .mu.m is adopted. The AlAs mole fraction is chosen to be so high as to make the absorbance of generated light small, e.g., 0.65 to 0.8. The difference in the crystal lattice spacing between the first epitaxial growth layer and the GaAs single crystal is only 0.14%, so that there is almost no fear that crystallographic defects occur due to the difference in lattice spacings.
Selection of an AlAs mole fraction of 0.8, for example, for this kind of p-type high AlAs mole fraction GaAlAs thick-film layer causes the surface of the layer to be easily oxidized due to high Al content, which inevitably results in a formation of an Al oxide film on the thick-film layer due to contact with oxygen in the environmental air by the time the growth of the afore-mentioned clad layers or active layer begin thereon. This in turn hinders the epitaxial growth of both of the clad layers and the active layer to a great extent. This hindrance means, for instance, an irregular growth interface or the degradation of the crystallinity due to occurrence of crystal defects. The following values of AlAs mole fractions, dopants, concentrations thereof and thicknesses are adopted for a p-type clad layer, an active layer and an n-type clad layer. The above-mentioned hindrance to epitaxial growth has a serious influence on the active layer which is an active layer for emittance of light. This hindrance causes the interface to be irregular between the active layer and the clad layer or causes the crystallinity of the active layer itself to be degraded, which causes the light emitting efficiency to be remarkably lowered.
______________________________________ (AlAs mole) (Thichkness) fraction) (Dopant) (Concentration) (.mu.m) ______________________________________ p-Type 0.8 Zn 1 .times. 10.sup.17 cm.sup.-3 approx. 50 Clad Layer Active 0.38 none -- 0.5 to 1 Layer (Non- Doped) n-Type 0.8 Te 2 .times. 10.sup.17 cm.sup.-3 approx. 30 Clad Layer ______________________________________
The above respective specifications of the two clad layers and active layer will now be briefly explained. Firstly, in order to provide a window effect (a clad layer will function as a window, passing photons having energy less than the band gap of the clad layer), the respective AlAs mole fractions of the two clad layers are determined in accordance with the wavelength of the light to be emitted from the active layer. The AlAs mole fraction of the active layer corresponds to the wavelength of the light desired to be emitted, and in this case, corresponds to the emission of red light which has wavelengths in the vicinity of 660 nm. The dopant level or concentration is selectively determined so that the light emission efficiency of the active layer may be maximized. In regard to the thickness of the epitaxial layer, the thickness of the active layer preferably must be small from the theoretical point of view. The most suitable value of the thickness of the active layer is selected in order to improve the crystallinity and to eliminate a harmful effect on the regularity of the interface of the epitaxial layer and the clad layer. The thickness of the clad layers is so selected as to improve crystallinity and to prevent troubles at the time of the device manufacturing processes. The above-mentioned thick-film layer may be grown so thick that it can function as a substrate. The epitaxial growth of this thick layer is suspended at a certain point when the thickness is enough for the purpose on the way of the epitaxial growth process. Further, the epitaxial growth of the two clad layers and the active layer are added therebetween on the substrate. Thus, the manufacturing processes of the GaAlAs light emitting semiconductor device is ordinarily performed on a discontinuity basis. Once a thick film layer is grown to be formed on a GaAs substrate and taken out of the furnace, it is difficult to prevent the surface of the thick-film layer from oxidation even if the GaAs substrate with the thick layer is protected in an atmosphere of an inert gas. When taken out of the furnace, an oxide film is formed on the thick-film layer. When two clad layers and an active layer therebetween are additionally grown onto the thick-film layer with the oxide film left thereon, an intervention of the oxide film in addition to the presence of the above-mentioned drawback causes an increased electric resistance to degrade the electric characteristics such as rise time. Accordingly, in order to eliminate these drawbacks a method is contrived wherein the oxide film is removed together with the substrate by a few micro-meters. Anyway, it frequently occurs that the oxide film keeps the growth surface from growing uniform. Another proposed method to eliminate said drawbacks is disclosed in Japanese Patent Unexamined Publication No. 62-14420, wherein a GaAs single-crystal thin film of, for example, 5 .mu.m thick is grown to form a protective film on the thick-film layer by use of a second solution for crystal growth so that the thin film can protect the thick-film layer from oxidation. Thereafter, the oxidation-resisting thin film is removed by melt-back treatment immediately preceding a next process step.
The present invention has been made in view of the fact that the protective film on the thick-film layer has a serious drawback in manufacturing a GaAlAs light-emitting semiconductor device having a double-hetero junction structure, the manufacture of which is the purpose of this invention, and that drastic changes are necessary for eliminating such a drawback. The reasons therefor are as follows.
Where a GaAs film or a GaAlAs film having only a low AlAs mole fraction of x&lt;0.05 is grown for protection as in the above-mentioned case, a contact of a solution for growing the protective film with a high AlAs mole fraction thick-film layer having such a ratio as Ga.sub.0.2 Al.sub.0.8 As causes an Al atomic fraction of a Ga solution and an AlAs mole fraction of the GaAlAs compound semiconductor single-crystal to be deviated remarkably each other from an equilibrium value, resulting in a serious unstable phase equilibrium between the solid phase and the liquid phase. More specifically, since the Al atomic fraction in the solution to be equilibrated with the AlAs mole fraction of 0.8 in the thick film is 0.022, when a Ga solution which is in phase equilibrium with a low Al containing GaAlAs layer to be deposited is allowed to contact a solid phase having an AlAs mole fraction of 0.8 and thereafter is subjected to cooling, the liquid phase causes the GaAlAs layer to begin its growth, but at the same time, the original solid phase is also vigorously eluted so as to equilibrate the AlAs mole fraction in the solid phase with the Al atomic fraction in the solution. In the case where both are largely deviated from the value of equilibrium, the solution should be maintained at a high degree of supersaturation so that, after its contact with the solid phase, crystal growth can be started at a rate higher than that at which the solid phase is eluted. However, it is difficult to control such high degree of supersaturation during the continuous growth process. For this reason, the growth interface becomes non-uniform and microscopically fluctuated.
In the technique disclosed in Japanese Patent Unexamined Publication No. 62-14420, a Ga solution without any solute is employed for the performance of a melt-back treatment. The reason for this is considered to lie in the respect that the solution for forming the p-type clad layer fails to be employed for melt-back treatment for the same reasons as mentioned above. However, even if the melt-back of the GaAs protective layer has been successfully performed, contact of an Al free Ga solution used for the melt-back treatment, with a high Al containing layer after the solution has completed melt back of the protective layer allows the phase equilibrium to be remarkably subjected to change, causing the elution of the solid phase and the deposition from the liquid phase to occur at the same time and thereby letting the interface be fluctuated. Therefore it is extremely difficult to prevent such fluctuations. The non-uniformity of the surface of the thick-film layer, which inherits at the time of deposition thereto of the GaAs protective layer, is memorized on the surface of the clad layer.
In addition, need of another solution for melt-back treatment undesirably makes the process more troublesome.
The influence which the phase equilibrium gives to growth interface in case of Ga.sub.1-x Al.sub.x As crystal growth and of melt-back process will be described.
In the case where the Ga.sub.1-x Al.sub.x As crystal is grown by the liquid phase epitaxial growth method (LPE method), growth of crystal is effected in general by using Ga as a solvent and GaAs and Al as a solute to thereby prepare a growth solution. In this case, the AlAs mole fraction x in the grown crystal is determined by the growth temperature and the Al atomic fraction in the growth solution as shown in FIG. 5. Namely, FIG. 5 graphically shows at each temperature the relationship between the Al atomic fraction X.sub.Al in the liquid phase (growth solution) and the corresponding AlAs mole fraction x in the solid phase (grown crystal) which is in phase equilibrium with the liquid phase. According to this graphic diagram, when it is desired to obtain a mixed crystal having an AlAs mole fraction x of 0.8 at a temperature of, for example, 900.degree. C., a liquid phase at a point A shown is necessary, so that growth of crystal starts to be effected from the Al mixed solution having an Al atomic fracion x.sub.Al =0.022. In other words, since the liquid phase epitaxial growth is effected in accordance with the phase equilibrium, the solution having an Al atomic fraction of x.sub.Al =0.022 and the Ga.sub.1-x Al.sub.x As mixed crystal having an AlAs mole fraction of x=0.8 can be said to be in phase equilibrium at 900.degree. C. Accordingly, the surface of the crystal grown from the solution by slow cooling process, or the surface of the crystal resulting from the melt-back treatment carried out by slow rising of temperature, show a good smooth one because crystal growth or melt-back is effected in a state of near phase equilibrium.
However, in the case where crystal growth or melt-back is effected with both the solid phase and liquid phase being kept largely out of equilibrium, for example, in the case where the saturated solution having almost no Al content as shown at a point B in FIG. 5 and the Ga.sub.1-x Al.sub.x As mixed crystal having an AlAs mole fraction of x=0.8 are caused to contact with each other at 900.degree. C., the protective layer starts to grow. On the other hand, however, the solid phase and the liquid phase act to diminish the discrepancy from the phase equilibrium, so that elution of Al from the Ga.sub.0.2 Al.sub.0.8 As mixed crystal takes place so as to supply Al to the liquid phase. In consequence, the interface between the solid and liquid phases become unstable. Thus, good crystal growth is unlikely to occur at the interface.